1. Field of the Invention
The disclosed technology relates to the field of characterization of doped semiconductors, and more particularly non-destructive optical measurement techniques for determining the active doping concentration profile of a semiconductor region.
2. Description of the Related Technology
The ITRS roadmap highlights the precise characterization of ultra-shallow junctions, formed by shallow doping of semiconductor regions, as one of the top challenges for sub-32 nm Si-CMOS technologies. Such a junction is typically characterized by a maximum active doping level N and a junction depth Xj and an abruptness S.
The accurate measurement of free carrier profiles in ultra-shallow junctions (USJ), such as the source and drain extension regions, is one of the major challenges of metrology in modern silicon Complementary metal-oxide-semiconductor technology. The used physical and electrical analytical techniques for determining the maximum doping level and junction depth, such as secondary ion mass spectrometry (SIMS), spreading resistance profiling (SRP), four point probe (FPP), or alternative candidates, such as scanning spreading resistance microscopy (SSRM) allow an accurate determination of this junction depth Xj. However these characterization techniques are destructive and quite slow, e.g. as samples have to be prepared, and therefore prevent any in-line measurement. There is still a clear absence of an accurate, fast, non-destructive technique.
Photomodulated optical reflectance (PMOR) is a widely used non-destructive and contactless technique to characterize in a qualitative way the doping profile of such a doped semiconductor region. It is a fully optical, hence non-contact, pump-probe technique. During measurement, a modulated-power pump laser is directed towards the doped semiconductor region to modify the refractive index profile thereof. This refractive index profile can be modified through generation of excess carriers in the sample, also known as the Drude effect, and/or by temperature effects of the sample under study. Simultaneously a probe laser is directed to this doped semiconductor region where it will be reflected depending on the refractive index profile. By coupling the reflected probe laser signal to a lock-in amplifier, only the variations in the reflectivity of the doped semiconductor sample induced by the modulated pump laser are measured. The probe laser thus measures, via reflection, the changes in refractive index induced by the pump laser.
PMOR is commonly used for monitoring implant dose in as-implanted (i.e. unannealed) silicon wafers, and hence has been the subject of much investigation in this field. On such as-implanted profiles, the variation in refractive index is due to the large increase in the temperature in the illuminated sample (dominant thermal component). This technique has also been studied extensively on box-like active doping profiles as obtained by chemical vapor deposition (CVD), where the measured signals are due mostly to the pump-generated excess carriers with only a weak contribution due to a mild increase in temperature (dominant plasma component).
An example of such PMOR technique is the Therma-Probe® technique (TP) described in “Non-destructive analysis of ultra shallow junctions using thermal wave technology” by Lena Nicolaides et al. in Review of Scientific Instruments, volume 74, number 1, January 2003. The TP technique is a high-modulation-frequency implementation of the PMOR technique.